Fractional distillation of multi-component streams to effect separation is a well known chemical engineering process and is used extensively in the chemical industry. It is well recognized that although distillation is widely used, it is also energy-intensive and often is the dominant cost in a distillation process. With rising energy costs, efforts have been made to enhance the efficiency of the distillation process through thermal coupling or through the use of heat pumps and the like. Representative art illustrating the enhancement of distillation efficiency via heat pumps or thermal coupling include the following:
An article entitled "Minimum Energy Requirements of Thermally Coupled Distillation Systems", AICHE Journal, Vol. 33, No. 4, (pp. 643-653, April 1987) discloses four different thermally coupled distillation systems consisting of distillation columns connected by liquid and vapor counter-current streams. One embodiment shows thermal coupling to a main column with a side arm column wherein a vapor is removed from the rectification zone in the main column and fed to an upper portion of the side column. A liquid stream from the side column then is returned as reflux to the rectification zone in the main column. Optionally, a liquid is removed from the stripping section of the main column and fed to a lower portion of the side column. The vapor is returned to the stripping zone of the main column. Another embodiment shows a thermally coupled system associated with a stripping column wherein liquid is removed from the main column and introduced to an upper portion of the stripping column. Lighter components are removed therefrom with the vapor from the stripping column being returned to the main column. Reboilers are associated with both the main column and stripping column to provide boilup. (Page 647)
An article entitled "Heat Integration of Distillation Columns Into Overall Processes", Chem. Engineering Science, Vol. 38, No. 8, pages 1175-1188 (1983), discloses energy enhancing techniques for the separation of multi-component systems in a multi-column distillation process. It was noted in a conventional method that reactor feeds were preheated with other process streams and steam before passing through a furnace. Steam was used as a heat source for the reboilers. The process incorporated the use of a distillation train feed in the reboiler of the first column for effecting vaporization of the liquid at the bottom, thus reducing the need for steam.
An article entitled "Distillation with Intermediate Heat Pumps and Optimal Side Stream Return", AICHE Journal, Vol. 32, No. 8, pages 1347-1359, (August 1986), discloses the separation of multi-component streams using a multicolumn distillation system. The term "heat pump" as conventionally used in these systems referred to the removal of heat from a location in the rectification section in the distillation column to the stripping section of the distillation column. One of the simpler techniques used in the prior art involved the movement of heat from the overhead vapor in a distillation system to the reboiler in an adiabatic column to effect an alteration of the internal reflux ratio. Examples of various methods of altering the internal reflux ratio involved by removing vapor from a column at a point above a feed plate, condensing that vapor fraction in a reboiler and returning it to an optimal location. Another process scheme involves removal of liquid from the stripping section of a column, vaporizing at the expense of compressed overhead vapor, and returning to an optimal point in the column.
U.S. Pat. No. 4,025,398 discloses a fractional distillation process wherein multiple columns are intercoupled to provide variable reboil and variable reflux so as to approach thermodynamically ideal fractionation. The system comprised a variable reboiler column and a variable reflux column wherein the variable reflux column was operated at a higher pressure and mounted at a lower level than the variable reboil column. Vapor was drawn from the variable reflux column, condensed at an upper level in the variable reboil stripping column and returned to the variable reflux column.
U.S. Pat. No. 4,234,391 discloses a continuous distillation apparatus incorporating separate stripping and rectifying sections in tandem, each of which are segregated into a plurality of vapor/liquid contact stages. In the process, the rectifying section of the column is operated at a higher pressure than the stripping section and this is achieved by compressing vapor from the stripping section prior to introducing the vapor into the rectifying section.
U.S. Pat. No. 4,605,247 discloses a process for the production of medium to high purity oxygen as well as other components contained in air. A triple pressure distillation process is developed in which the low pressure column has an argon stripping section and a rectification section reboiled by the high pressure column. At least one latent heat exchange is made from an intermediate height of the low pressure column with an intermediate height in a moderate pressure column. Latent heat exchanges are used to insure high reboil through the argon stripping section of the low pressure column.